1. Field of the Invention
The present invention relates to a methacrylimide-containing polymer having an excellent heat resistance, transparency, and resistance to discoloration under heating with a lapse of time, and to a resin composition comprising this methacrylimide-containing polymer and having an excellent resistance to discoloration under heating with a lapse of time.
2. Description of the Related Art
A methyl methacrylate polymer has not only an excellent transparency but also excellent mechanical properties and weatherability, and therefore, this polymer has been used as a high-performance plastic optical material or decorative material. Recently, the development of uses of this polymer in the fields of short-distance communication and optical sensors has been investigated.
Nevertheless, since the heat distortion temperature of the methyl methacrylate polymer is about 100.degree. C. and the heat resistance is too low, use of this polymer is considerably restricted in some fields where a heat resistance is required, and therefore, there is an urgent need for an improvement of the heat resistance of this polymer.
Imidization of a methyl methacrylate polymer is known as the means for improving the heat resistance of the methyl methacrylate polymer. For example, there have been proposed (1) a process in which a polymer of acrylic acid, methacrylic acid or an ester thereof is reacted under heating with a primary amine, ammonia or a compound capable of generating a primary amine or ammonia in the presence of a solvent (U.S. Pat. No. 2,146,209, German Patent No. 1,077,872 and German Patent No. 1,242,369), (2) a process in which a methyl methacrylate polymer is reacted with a primary amine in the presence of water (U.S. Pat. No. 3,284,425), and (3) a process in which an acrylic polymer is reacted with ammonia or a primary amine in an extruder (U.S. Pat. No. 4,246,374).
In the process (1), since the boiling point of the used solvent is high, it is difficult to completely separate the solvent from the imidized polymer on a commercial scale, and therefore, the obtained imidized polymer is colored and the transparency of the formed polymer is poor. In the process (2), since the reaction is carried out in the presence of water, when a partially imidized polymer is required, hydrolysis of the methyl methacrylate segments occurs, and therefore, it is difficult to obtain an imidized polymer having a desired heat resistance, and to obtain a uniform imidization. Furthermore, in the process (3), since the imidization reaction occurs between a polymer having a high viscosity and a gaseous imidizing substance, it is difficult to obtain a uniform imidization reaction, and therefore, it is difficult to obtain a uniformly partially imidized polymer.
Accordingly, the imidized polymers obtained according to the above-mentioned processes have an improved heat resistance, but if it is intended to prepare the polymers on a commercial scale, the transparency is poor or the imidization reaction becomes uneven, and therefore, these processes are not practically usable.
Japanese Unexamined Patent Publication No. 60-210606 discloses a process for the preparation of polymethacrylamide having an excellent transparency, in which an incorporation of iron from an extruder is prevented by reprecipitating a reaction product, by using a solvent and filtering and drying the reaction product. This process is complicated, however, and thus is not suitable for commercial working. Moreover, since the polymer obtained by reprecipitation is powdery, an extreme yellowing of the polymer occurs at the heat molding step.
Japanese Unexamined Patent Publication No. 62-187705 discloses a process wherein production of polymethylmethacrylate and imidization thereof are continuously carried out. The polymethyl methacrylimide obtained by this process and that obtained by the process of the above-mentioned Japanese Unexamined Patent Publication No. 60-210606 have a total luminous transmittance of 93 to 94%, but control of the imidization ratio is difficult in these processes, and although polyacrylamide having a narrow imidization ratio distribution can be obtained if sampling is carried out for a short time, the imidization ratio often changes if the operation is continuously carried out for from several hours to scores of hours. Therefore, if products prepared by continuously carrying out the operation for a long time are mixed and used, the transparency is poor because of a broad imidization ratio distribution.
Furthermore, a resin composition formed by mixing and blending the above-mentioned methacrylimidecontaining polymer with another thermoplastic polymer has a problem in that an extreme yellowing of the molded article occurs and this yellowing is increased due to a deterioration caused by heating with a lapse of time, with the result that the commercial value is drastically reduced.